I have never used FFFFG in my revolvers. Every time I try to order it, the salesman figures I have a stutter and hands me FFG.
But seriously, I haven't heard that FFFFG has a significant advantage over FFFG or FFG. I use a well-greased felt wad twixt ball and powder, seated firmly on the powder before seating the ball. This keeps fouling to a minimum.
In fact, I've popped the barrel of my Colt 1851 Navy and shown smokeless powder friends, who were amazed that there was but a fine layer of gray ash in the barrel, not mini-briquets of crud like they'd always heard.
I use FFFG and FFG in my .36 and .44 revolvers.
The Ruger Old Army is a very strong revolver; probably the strongest cap and ball revolver ever made. I'm at work and have no reference to pressures generated by FFFFG in revolvers. I'll see what I can find at home.
I think that FFFFG would work fine in the .31, and perhaps the .36 if it's all you have, but I wouldn't want to use it in a .44 with its greater chamber volume.
I'm no ballistician, but I'm always amazed at how the smallest changes in components often bring out the worst nature of the beast we call gunpowder (whether the ancient stuff or smokeless).
And one thing that can rouse that beast is the volume in which gunpowder is confined and must work.
If you decide to use FFFFG, let us know what the results are. Sounds like an interesting post. I would confine FFFFG experiments only to the Ruger Old Army. The use of FFFFG may generate pressures that strain any other model, especially an original.